Inert gas shield for atomic absorption or flame emission spectrometer

ABSTRACT

This invention relates to the use of an inert gas shield enveloping the flame of either an atomic absorption spectrometer or a flame emission spectrometer to improve the response of cations. Both the sensitivity and the precision of analysis of refractory metals and rare earth oxides are improved by the use of an inert gas shield (e.g., argon) to surround the flame in which the cations are excited. Argon gas is allowed to flow about the flame so as to form an envelope in such a manner as to stabilize the flame, retard oxidation of the ions, and increase concentration of elements in the ground state. Spatial distribution of &#39;&#39;&#39;&#39;free atoms&#39;&#39;&#39;&#39; in the flame has been found to depend not only upon the flame stoichiometry but also on the geometry of the flame, as well as upon a nonchemically active flame edge. The argon envelope gives improved flame geometry by reducing the flame waving and gives a nonchemically active flame edge.

United States Patent Cummings et al.

[451 Oct. 17, 1972 INERT GAS SHIELD FOR ATOMIC ABSORPTION OR FLAME EMISSION SPECTROMETER [72] Inventors: John P. Cummings; Timothy J.

Gomoil, both of Toledo, Ohio [73] Assignee: Owens-Illinois, Inc.

[22] Filed: Jan. 15, 1971 [2]] Appl. No.: 106,824

[52] U.S. Cl. ..239/299, 239/568, 239/597 I [51] Int. Cl. ..B05h 1/28 [58] Field of Search ..239/290-301, 552, 239/568, 597

[56] References Cited UNITED STATES PATENTS 3,526,362 9/1970 Jackson ..239/290X 2,005,308 6/1935 Anderson ..239/296 645,416 3/1900 Bush ..239/290 3,086,851 4/1963 Wagner ..239/291 X 3,409,233 11/1968 Kiernan ..239/597 X 3,434,668 3/1969 Boling ..239/568 X Primary Examiner-M. Henson Wood, Jr. Assistant Examiner-John J. Love Attorney-Alan J. Steger and E. J. Holler [57] ABSTRACT This invention relates to the use of an inert gas shield enveloping the flame of either an atomic absorption spectrometer or a flame emission spectrometer to improve the response of cations. Both the sensitivity and the precision of analysis of refractory metals and rare earth oxides are improved by the use of an inert gas shield (e.g., argon) to surround the flame in which the cations are excited. Argon gas is allowed to flow about the flame so as to form an envelope in such a manner as to stabilize the flame, retard oxidation of the ions, and increase concentration of elements in the ground state. Spatial distribution of free atoms" in the flame has been found to depend not only upon the flame stoichiometry but also on the geometry of the flame, as well as upon a nonchemically active flame edge. The argon envelope gives improved flame geometry by reducing the flame waving and gives a nonchemically active flame edge.

1 Claim, 5 Drawing Figures PATENTEDBBT 1 m2 I 3.698.643

SHEU 3 BF 4 INVENTORS JOH vmm oes BY TMOTHQ GomoLk INERT GAS SHIELD FOR ATOMIC ABSORPTION OR FLAME EMISSION SPECTROMETER BACKGROUND OF THE INVENTION This invention relates to a system employing either an atomic absorption spectrometer or a flame emission spectrometer and, more particularly, to a spectroscopy system which utilizes an inert gas shield to surround the burner flame to improve the response of the cations.

Flame atomic absorption spectroscopy involves the measurement of the absorption of radiation by the ground state atoms of the element to be determined. The source of radiation is a hollow cathode or electrodeless discharge tube selected to emit the spectrum of the element being determined. In the atomizerburner system the solution to be analyzed is drawn up a capillary tube and converted by means of a stream of compressed air or nitrous oxide to a fine spray which, after condensation of the larger droplets, is mixed with acetylene or hydrogen and burned in a long flame (laminar flow) at a titanium burner head. The radiation from the lamp as a constant output traverses the flame where some is absorbed by ground state atoms of the element to be determined. The radiation remaining after traversing the length of the flame enters a monochromator, which has been set at the resonance line of the element being determined, and then falls on a photomultiplier. The radiation from the lamp (chopped at 285 cps), is amplified by an A. C. amplifier, and the output presented on a meter, recorder, or (nixie) tubes.

Flame emission spectroscopy involves the measurement of radiation emitted by an atom in the excited state. Flame emission spectroscopy utilizes the same equipment as atomic absorption spectroscopy except there is no hollow cathode tube needed since the source of radiation is the thermally excited atoms in the flame. Flame emission spectroscopy performed on an instrument basically built for atomic absorption spectroscopy usually utilizes a chopper between the flame and monochromator to synchronize the signal from the flame with the modulating frequency of the amplifier.

The results obtained from either the flame atomic absorption spectroscopy method or the flame emission spectroscopy method may be plotted to generate a curve for various levels of the element being analyzed. Unfortunately, the results obtained to date have been relatively vague and afforded only rough approximations of the amounts of a given element present, especially at low concentration levels.

Therefore, there has been long-felt need for an improved flame atomic absorption spectroscopy system or a flame emission spectroscopy system which would provide much more accurate readings as to the amount present of the element being analyzed, especially at low concentration levels.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an inert gas shield which envelopes the flame of either an atomic absorption spectroscopy system or a fame emission spectroscopy system to improve the response of the cations. Both the sensitivity and precision of analysis of refractory metals and rare earth oxides are improved by the use of the inert gas shield (e.g., argon) to surround the flame in which the cations are excited. Argon gas is allowed to flow about the flame so as to form an envelope in such a manner as to stabilize the flame, retard oxidation of the ions, and increase concentration of the elements in the ground state. Spatial distribution of free atoms in the flame has been found to depend not only upon the flame stoichiometry but also on the geometry of the flame, as well as upon a non-chemicallyactive flame edge. The argon envelope gives improved flame geometry by reducing the flame waving and gives a non-chemically active flame edge.

It has been found that the use of an inert gas shield enveloping the flame of either an atomic absorption spectroscopy system or a flame emission spectroscopy system results in vastly improved readings as to the amounts of a given element present at very low concentration levels. I

Other objects, features, and advantages of the subject invention will become obvious to those skilled in the art upon reference to the following detailed description of the invention and the drawings illustrating a preferred embodiment thereof.

In The Drawings:

FIG. 1 is a schematic perspective view of a spectroscopy system, which may function as either an atomic absorption spectroscopy system or a flame emission spectroscopy system, and which utilizes the unique inert gas shield of this invention to envelope the flame in its burner.

FIG. 2 is a perspective view of the unique burner head of this invention which incorporates an inert gas shield to envelope the flame emanating therefrom.

FIG. 3 is an end view of the burner head of FIG. 2.

FIG. 4 is a perspective assembly view of the component parts of the burner head of FIG. 2, and,

FIG. 5 is a graph showing the readings obtained by a flame emission spectroscopy system functioning both with and without an argon gas shield.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawings, a spectroscopy system, which is adapted to function as either a flame atomic absorption spectroscopy system or a flame emission spectroscopy system, is illustrated in FIG. 1 and designated generally by the numeral 10. The spectroscopy system 10 includes a number of component parts which are mounted adjacent to or directly on an optical rail 12. Mounted at one end of the optical rail 12 is a multiple lamp rack 14, which is adapted to hold a plurality of hollow cathode lamps such as those illustrated at 16, 18, 20, and 22. A hollow cathode lamp 24 has been selected in this instance and is positioned on a mounting 26, which is adapted for sliding movement on optical rail 12. These hollow cathode lamps 16-24 are of the standard type which consist of a metal rod anode and a hollow cylindrical cathode made of or lined with the metal whose spectrum is desired. The electrodes are mounted in a sealed tube fitted with a glass or silica window and filled with argon, helium, or neon at low pressure. When the lamp is connected to a power supply, the discharge taking place is concentrated inside the hollow cathode, and bombardment by the raregas ions causes free atoms of the metal to be sputtered off the cathode. These atoms are excited by collision with the rare-gas atoms and emit a strong sharp-line spectrum of the metal. Thus, the lamp selected will depend upon the metal whose spectrum is desired to be analyzed. The lamp rack 14, which holds lamps 1622, provides current stablization for the operating lamp 24 and also provides current for partial stablization for the warm-up operation of the other lamps 16-22. Thus, the lamps 16-22 are kept in a warmed-up, ready-tooperate condition while the lamp 24 is operating to detect the presence of a given element within the sample being tested. A first lens 28 is mounted by means of a base 30, which is adapted for sliding movement on optical rail 12, adjacent to the hollow cathode lamp 24. This first lens 28 is adapted to form an image of the hollow cathode lamp 24 above the burner section 32. A second lens is mounted on a base 36, which also is adapted for sliding movement on the optical rail 12 and positioned downstream of the burner section 32 to focus the image onto a monochromator 38.

The burner section 32 includes a burner head 40, spray chamber 42, and an atomizer 44. The solution to be analyzed resides within a breaker 46 which is connected by means of a capillary tube 48 to the atomizer 44. Compressed air is contained within a suitable tank 50 and enters into the atomizer 44 to thereby draw the solution to be analyzed up through the capillary tube 48 where it is converted to a fine spray within the spray chamber 42. The resulting spray impinges on a glass bead (not shown) which reduces its velocity so that the larger droplets condense out and are eliminated from the spray chamber 42. The fine droplets remaining are mixed with an appropriate combustion gas (e.g., hydrogen, propane, or acetylene) in the spray chamber 42 and are then burned in a flame within the burner head 40. The compressed air is maintained at about psi as it is forced into the atomizer 44. It should be noted that the burner section 32 is of a standard commercially available type such that further details of its component parts would be obvious to one skilled in the art. The details of the burner head portion 40, which incorporates the unique inert gas shield of this invention, will be discussed in connection with FIGS. 2-4.

The remaining element of the spectroscopy system 10 is the monochromator 38, which is positioned at the end of the optical rail 12 on a suitable monochromator base 52. The monochromator 38 is a standard commercially available model, which is adapted to receive the image which has been focused by the second lens 34 and feed information to a photomultiplier (not shown) from which a readout can be made to determine whether a given element is present within the solution being analyzed.

The component parts of the burner head 40, which incorporates the unique inert gas shield of this invention, may best be seen by reference to FIG. 4. The burner head 40 includes a standard commercially available burner plate 54, which is connected by means of conduit 56 to the spray chamber 42. The burner plate 54 has laminar flame slit 58 formed in its upper surface which connects directly to the open channel 56. A pair of recessed slots 60 are formed in the upper surface of burner plate 54 on opposite sides of laminar flame slit 58.

The unique feature of this invention is embodied in a pair of gas shield shroud members 62 and 64 which fit over the burner plate 54 and have recessed portions 66 on an inner edge adjacent to the laminar flame slit 58. These shrouds 62 and 64, when placed on top of burner plate 54, are fastened together by means of bolts 68, sleeves 70, and nuts 72. Each of the shrouds 62 and 64 have a pair of gas conducting conduits 74 and 76, respectively, connected to opposite ends thereof. Thus, when the shrouds 62 and 64 are fastened in contact with the burner plate 54 as shown in FIGS. 2 and 3, the area under the shrouds 62 and 64 is completely sealed off with the exception of the conduits 74 and 76 and the recessed portion 66 adjacent to the laminar flame slit 58. Thus, gas entering through conduits 74 and 76 may exit from beneath the shrouds 62 and 64 only through the recessed portion 66 adjacent to the laminar flame slit 58. Referring now to FIG. 1, it can be seen that the conduits 74 and 76 are connected by means of suitable tubing 78 and 80 to a beaker 82, which in turn is connected by means of tube 84 to an inert gas tank 86. The inert gas tank 86 has a suitable flow control valve 88 thereon to control the flow of inert gas to the beaker 82. Thus, inert gas, such as argon, is allowed to flow from the tank 86 into the beaker 82 and then by means of tubes 78 and 80 is fed to the opposite ends of shrouds 62 and 74. The inert gas then exits from the shrouds 62 and 64 through the recessed portions 66 to thereby form a surrounding shield around the flame which is present in the laminar flame slit 58. Thus, it is the addition of the inert gas shielding system to the burner plate 54 which results in the unique concept of this invention wherein the burner flame is shielded so that the spectroscopy system provides much more accurate results in determining the presence of a given element in a solution being analyzed.

If it is desired to operate the system 10 as shown in FIG. 1 as a flame atomic absorption spectroscopy system, a suitable hollow cathode lamp, such as that shown at 24, is selected to correspond to the element whose presence it is desired to determine. The solution to be analyzed is placed in beaker 46, drawn up capillary 48, and converted by means of compressed air from tank into a fine spray which is ignited with a suitable fuel (such as acetylene or hydrogen) and burned in the burner head 40 to appear as a flame in laminar flame slit 58. Simultaneously, an inert gas such as argon is delivered to the flame shield shrouds 62 and 64 by way of beaker 82 and tank 86 to envelop the flame as it passes through laminar flame slit 58. The radiation from the lamp at a constant output traverses the flame where some is absorbed by ground state atoms of the element to be determined. The radiation remaining after traversing the length of the flame is focused by the second lens 34 and enters the monochromator 38 which has been set at the resonance line of the element being determined and then falls on a photomultiplier (not shown). The output data is then recorded to indicate whether the given element sought is present within the solution being analyzed.

If, on the other hand, the system designated by the numeral 10 is desired to be operated as a flame emission spectroscopy system, the hollow cathode lamps and first lens are unnecessary, since the source of radiation is the thermally excited atoms in the flame. In this system, the solution to be analyzed is ignited at the burner head and the resulting flame shielded with an inert gas shield so that the resulting radiation is focused by means 34 into the monochromator 38 for analyzation and recording of the elements present.

A sample of such results from such a flame emission spectroscopy system used to analyze for the presence of erbium oxide (E is illustrated in the chart of FIG. 5. The vertical axis is divided into chart divisions and the horizontal axis divided into parts per million of Er 0 The lower curve on this graph is a plot of selected points of parts per million of Er 0 using a flame emission spectroscopy system without the argon shield. The upper curve is a plot of the results obtained which shows the parts per million of Er 0 indicated by a flame emission spectroscopy system including the unique argon gas flame shield of the subject invention. lt can be seen from the graph of FIG. 5 that considerably more accurate results can be obtained, particularly approximations of parts per million present at low concentration levels when the system utilizes an argon gas flame shield.

Thus, it can be seen from the preceding description that the use of the unique argon shield in this invention in a spectroscopy system improves the sensitivity and precision of the readings obtained from such a system. in other words, the use of the inert gas shield greatly increases the ability of a spectroscopy system to determine a small change in concentration of solute at any concentration level. For example, the shielding device of this invention when rare earths were being analyzed increased the gain in sensitivity of Neodymium and Samarium at least three times. The increase noted for refractory materials, such as aluminum, was in the range of percent. The increase in detection limits was also notable, especially in the determination of other refractory materials such as silicon and barium.

Thus, from the preceding description, it should be apparent that the utilization of the unique inert gas flame shielding device and method of this invention in either a flame atomic absorption spectroscopy system or a flame emission spectroscopy system greatly in- 6 crease the sensitivity of the analyzation results obtained from such a system.

We claim:

1. In combination:

a laminar flow burner for use in a flame spectroscopy system, said burner having an upper burner surface, a laminar flame slit formed in said upper burner surface and through which said laminar flame emanates from said burner, and a pair of recessed slots formed in said upper burner surface on opposite sides of said laminar flame slit;

' and an inert gas shield adapted to cooperate with said burner to distributea uniform flow of inert gas to envelop and shield the laminar flame emanating from said laminar flame slit in said upper burner surface, said inert gas shield including a source of supply of inert gas, a pair of hollow shroud members having at least two closed sides, closed ends, and one partially open side, each of said shroud members being mounted on and attached to the upper burner surface in overlying relationship with said recessed slots in the upper burner surface on opposite sides of said laminar flame slit so that the partially open side of each of said shrouds is in direct communication with one of the recessed slots in said upper burner surface and the closed sides and closed ends of said shrouds being in engagement with the upper burner surface to define a pair of inert gas distribution chambers formed by the hollow spaces within said shrouds and the recessed slots in said upper burner surface, each of said shrouds including inert gas inlet conduits at each end thereof connecting said inert gas distribution chambers to said source of supply of inert gas, and each of said shrouds having a longitudinally extending portion of one of its closed sides removed in a location overlying the recessed slots in the upper burner surface to form a pair of inert gas outlet apertures in juxtaposition to the full length of said laminar flame slit. 

1. In combination: a laminar flow burner for use in a flame spectroscopy system, said burner having an upper burner surface, a laminar flame slit formed in said upper burner surface and through which said laminar flame emanates from said burner, and a pair of recessed slots formed in said upper burner surface on opposite sides of said laminar flame slit; and an inert gas shield adapted to cooperate with said burner to distribute a uniform flow of inert gas to envelop and shield the laminar flame emanating from said laminar flame slit in said upper burner surface, said inert gas shield including a source of supply of inert gas, a pair of hollow shroud members having at least two closed sides, closed ends, and one partially open side, each of said shroud members being mounted on and attached to the upper burner surface in overlying relationship with said recessed slots in the upper burner surface on opposite sides of said laminar flame slit so that the partially open side of each of said shrouds is in direct communication with one of the recessed slots in said upper burner surface and the closed sides and closed ends of said shrouds being in engagement with the upper burner surface to define a pair of inert gas distribution chambers formed by the hollow spaces within said shrouds and the recessed slots in said upper burner surface, each of said shrouds including inert gas inlet conduits at each end thereof connecting said inert gas distribution chambers to said source of supply of inert gas, and each of said shrouds having a longitudinally extending portion of one of its closed sides removed in a location overlying the recessed slots in the upper burner surface to form a pair of inert gas outlet apertures in juxtaposition to the full length of said laminar flame slit. 